The present invention relates to an oxygen concentration detecting element useable for an oxygen sensor for sensing oxygen concentration in exhaust gas.
Generally, automobiles are equipped with an oxygen sensor in an exhaust pipe coupled to an engine. The oxygen sensor is adapted for sensing oxygen concentration in exhaust gas emitted from the engine. An air/fuel ratio is feedback controlled to a stoichiometric ratio, i.e., 14.7, on the basis of the oxygen concentration sensed by the oxygen sensor.
FIG. 5 shows a longitudinal cross-section of an oxygen concentration detecting element of a related art. As illustrated in FIG. 5, oxygen concentration detecting element 100 includes base member 101, solid electrolyte layer 102 disposed outside base member 101, and porous layer 103 disposed on an outer surface of base member 101. Reference electrode 104 is disposed on an inner surface of solid electrolyte layer 102. Detecting electrode 105 is disposed on an outer surface of solid electrolyte layer 102. Solid electrolyte layer 102, reference electrode 104 and detecting electrode 105 cooperate to form oxygen detecting unit 120. Dense layer 106 with oxygen introduction window 106A covers outer surfaces of solid electrolyte layer 102 and detecting electrode 105 except a part of the outer surface of detecting electrode 105 which is exposed through oxygen introduction window 106A. Protecting layer 107 is disposed on an outer surface of dense layer 106 and the exposed part of the outer surface of detecting electrode 105. A flow of gas to be measured, for instance, an exhaust gas flowing from an engine into an exhaust pipe is introduced to the outside of dense layer 106 and protecting layer 107.
Base member 101 is constituted of cylindrical solid core rod 110, heater pattern 111 formed on an outer circumferential surface of core rod 110, and heater insulating layer 112 disposed on the outer circumferential surface of core rod 110 so as to cover heater pattern 111. Heater insulating layer 112 is made of an electrically insulating material and formed by printing. Thus, oxygen detecting unit 120 is overlaid on an outer surface of heater pattern 111.
Reference electrode 104 and detecting electrode 105 are made of a material having electrical conductivity and oxygen permeability. Reference electrode 104 and detecting electrode 105 have integrally formed leads, respectively. Only the lead of detecting electrode 105 is indicated at 113 in FIG. 5. An output voltage generated between reference electrode 104 and detecting electrode 105 is taken out through leads 113 and measured. Dense layer 106 is made of a material preventing oxygen in the gas to be measured from permeating therethrough. Protecting layer 107 is made of a material permitting oxygen in the gas to be measured to permeate therethrough but preventing harmful gases in the gas to be measured from permeating therethrough.
Operation of oxygen concentration detecting element 100 is explained below. When heater pattern 111 is energized to generate heat, the heat generated is transmitted to oxygen detecting unit 120 through heater insulating layer 112. Solid electrolyte layer 102 of oxygen detecting unit 120 is activated by the heat transmitted. Oxygen in the gas to be measured permeates through protecting layer 107 and detecting electrode 105 and reaches the outer surface of solid electrolyte layer 102. Oxygen in the atmosphere, acting as a reference, permeates through porous layer 103 and reference electrode 104 and reaches the inner surface of solid electrolyte layer 102. When there is a difference in oxygen concentration between the inner and outer surfaces of solid electrolyte layer 102, oxygen ions are transmitted through solid electrolyte layer 102 to produce electromotive force between reference electrode 104 and detecting electrode 105 depending upon the difference in oxygen concentration. As a result, an output voltage varying corresponding to the difference in oxygen concentration is obtained.
Japanese Patent Application First Publication No. 7-27737 describes such an oxygen concentration detecting element as explained in the related art.